1. Field of the Invention
The present invention relates to a toner used for a developer for developing an electrostatic latent image formed by an image forming method such as electrophotography, electrostatic recording methods and electrostatic printing methods. In addition, the present invention relates to an external additive for use in the toner, and to an image forming method using the toner.
2. Discussion of the Related Art
Developers used for developing an electrostatic latent image formed by an image forming method such as electrophotography, electrostatic recording methods and electrostatic printing methods are classified into two component developers consisting of a carrier and a toner, and one component developers which include only a toner and do not include a carrier. The one component developers are also classified into one component magnetic developers and one component non-magnetic developers. In addition, toners are also classified into positively-charged toners and negatively-charged toners.
In order to improve the fluidity and charge properties of toner, an inorganic powder such as metal oxides is typically included in a toner while the inorganic powder is mixed with the toner particles. Such an inorganic powder is called as an external additive. As the external additive, silicone dioxide (i.e., silica), titanium dioxide (titania), aluminum oxide (alumina), zinc oxide, magnesium oxide, cerium oxide, iron oxide, copper oxide, tin oxide and the like metal oxides are known.
In order to improve the hydrophobic property of the surface of such inorganic powders and the charge properties thereof, various methods have been proposed. In particular, methods in which silica powders are reacted with an organic silicon compound such as dimethyldichlorosilane, hexamethylenedisilazane and silicone oils to substitute the silanol groups on the surface of the silica powders with an organic group are popularly used.
Among these organic silicon compounds, silicone oils are preferable as the hydrophobizing agent because of having good hydrophobic property and low surface energy, thereby imparting good transferability to the resultant toner. Therefore various proposals regarding silicone oils have been made. For example, Japanese Laid-Open Patent Publications Nos. 7-271087 and 8-29598 have disclosed developers which include an additive having a specific carbon content and a specific silicone oil content to improve the charge stability of the resultant developer under high humidity conditions.
However, the developers including such additives including a silicone oil have the following drawbacks:
(1) the resultant images tend to have background fouling, and omissions in the edge parts and/or center parts of character images, line images and dot images, which omissions are caused by unsatisfactory image transfer from an image bearing member to a receiving material; and
(2) when an image is formed on a receiving material having a rough surface, the image tends to have omissions which are typically observed in the recessed portions of the surface of the rough receiving material.
In attempting to remedy the drawbacks, Japanese Laid-Open Patent Publication 11-212299 discloses an external additive in which a specific amount of a liquid silicone oil is included in a particulate inorganic material. However, the drawbacks cannot fully remedied.
On the other hand, currently, a need exists for copiers and printers which can produce images having qualities as high as those of the prints produced by offset printing methods at a recording speed as high as that of the offset printing methods. When images are formed on only one side of receiving material, fairly high speed copying or printing can be achieved. However, when images are formed on both sides of a recording paper, the copying or printing speed is much slower than that in the one-side printing. In particular, when a double-sided copy is produced by forming and fixing a toner image on one side of a receiving material and then forming and fixing another toner image on the other side of the receiving material, the recording time is about twice the recording time needed for producing a one-side copy.
Therefore, an image forming method, in which toner images are continuously transferred on both sides of a receiving material by two image bearing members which are arranged at different positions in the vertical direction as shown in FIG. 1 and the thus prepared double-sided toner images are then fixed at the same time has been proposed.
The image transfer method of the image forming method is classified into two types with respect to the method for driving the image bearing members. One of the types is to drive the image bearing members using a motor or a belt (this method is hereinafter referred to as a motor-drive method). The other is to drive the image bearing members using friction between the image bearing members and a receiving material without providing a rotation mechanism (hereinafter referred to as a paper-drive method).
In the motor-drive method, it is difficult to control the timing of transferring toner images on both sides of a receiving material. In order to accurately transfer images on both sides of a receiving material, the image forming apparatus has to have precise and complex mechanisms, resulting in increase of manufacturing costs and enlargement in size of the image forming apparatus. In particular, in multiple color image forming apparatus a problem of difference in positions between the transferred color toner images (hereinafter referred to as color image offset), in which color toner images constituting a color image do not have accurate positions in one side of a receiving material, occurs as well as a problem of variation of transfer timing of a color image (hereinafter referred to as transfer timing variation), in which an entire color image is formed on an undesired position of a receiving material. The costs of providing a mechanism for preventing these problems are much greater than those needed for monochrome image forming apparatus.
The paper-drive method will be explained referring to FIG. 1 which illustrates a typical embodiment of a paper-drive type image transfer mechanism.
In this method, a receiving material 1 is contacted with an image bearing member 2a on which a toner image 5a is formed to drive the image bearing member 2a while the toner image 5a is transferred on one side of the receiving material 1 by a transfer member 3a. In addition, the receiving material 1 is contacted with another image bearing member 2b on which a toner image 5b is formed to drive the image bearing member 2b while the toner image 5b is transferred on the other side of the receiving material 1 by a transfer member 3b. 
In the paper-drive method, image bearing members are driven by friction and/or electrostatic force between the image bearing members and a receiving material. Namely, the image bearing members are driven by the receiving material, and therefore, it is relatively easy to time the development and transfer operations with the feeding operation of the receiving material compared to the former method, and thereby the problems such as the color image offset and transfer timing variation hardly occur. Therefore the image forming apparatus can be simplified.
However, since the receiving material is contacted with the image bearing member, toner images sandwiched by the receiving material and the image bearing member receives a large pressure by the receiving material. Therefore the adhesion of the toner images to the image bearing member increases, and thereby omissions tend to be produced in the resultant transferred toner images, particularly in edge parts or center parts of the transferred character images, line images and dot images.
In attempting to solve this image omission problem, toners including an additive such as particulate inorganic materials e.g., silica and titanium oxide, have been proposed. However, such toners cannot fully solve the problem. Namely, when such an additive is included in a toner in a large amount to solve the image omission problem, the resultant toner images have poor fixing properties. Therefore it is difficult to prepare a toner capable of producing images having good image qualities (i.e., good images without image omissions) without deteriorating good fixing properties.
Because of these reasons, a need exists for a toner which can produce images having good image qualities and good fixing property even when used for the paper-drive image forming method.
Accordingly, an object of the present invention is to provide a toner which can produce images having good image qualities and good fixing property without causing image omissions even when used for paper-drive image forming method.
Another object of the present invention is to provide an external additive for the above-mentioned toner.
Yet another object of the present invention is to provide an image forming method by which images having good image qualities and good fixing property can be stably produced without causing image omissions even when images are formed for a long period of time.
To achieve such objects, the present invention contemplates the provision of a toner including a binder resin, a colorant and an external additive, wherein the external additive includes a particulate inorganic material and a silicone oil, and wherein the silicone oil has a free silicone degree of from 10 to 70%. At this point, the free silicone degree is defined as follows:
Free silicone degree (%)=(Wfs/Ws)xc3x97100 
wherein Wfs represents the weight of the free silicone oil and Ws represents the weight of the silicone oil present on the inorganic material.
The free silicone degree is preferably from 30 to 50%. In addition, the inorganic material is preferably silica, titanium oxide or a combination thereof.
The inorganic material is preferably treated by the silicone oil upon application of heat. In addition, the toner preferably has a spherical degree not less than 0.93, and a weight average particle diameter not greater than 15 xcexcm.
Preferably, the toner is a combination of a cyan toner, a magenta toner, a yellow toner and a black toner, and at least one of the toners is the toner mentioned above.
In another aspect of the present invention, an external additive for a toner is provided which includes a particulate inorganic material and a silicone oil, and wherein the silicone oil has a free silicone degree of from 10 to 70%.
In yet another aspect of the present invention, a developer is provided which includes a magnetic carrier and the toner of the present invention.
In a further aspect of the present invention, an image forming method including the steps of forming a toner image on an image bearing member with a developer including the toner of the present invention; transferring the toner image on a receiving material upon application of pressure; and then heating the toner image on the receiving material to fix the toner image. The toner image forming step may be repeated plural times to form a full color image on the image bearing member or an intermediate transfer medium. The full color image is then transferred on the receiving material. Alternatively, the toner image forming step may be performed using plural image bearing members to form respective color images thereon.
The image transferring step preferably uses a paper-driven image transferring method, and the image fixing step preferably uses a non-contact heat fixing method.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.